1. Field of the Invention
The present invention concerns a method for adjustment of a B1 field in a magnetic resonance apparatus, and a method for generation of a database for this purpose.
2. Description of the Prior Art
In the operation of a magnetic resonance apparatus, the examination subject is exposed to radio-frequency (RF) energy, in the form of RF pulses. The RF energy causes the nuclear spins in the examination subject that have been previously aligned by a strong, static magnetic field, to be deflected from the aligned (equilibrium) orientation. The precessing nuclear spins, as they return to their equilibrium position, cause an RF signal to be emitted from the subject, which is detected and analyzed to generate an image of the subject. The electromagnetic field that is produced by the emitted RF energy is referred to as the B1 field.
Problems occur in the adjustment of the homogeneity of the B1 field due to the use of increasingly strong magnetic field strengths in magnetic resonance tomography systems and the increasing excitation frequencies associated with the use of such strong fields. Inhomogeneous image brightnesses and image contrasts are the result. These are dependent on the structure of the measurement subject to be examined, for example on the anatomy of a patient to be examined.
Inhomogeneous image brightnesses dependent on the patient cannot be reduced or avoided by a uniformly designed radio-frequency coil.
It is possible to use a transmission antenna arrangement (known as a “Tx array”) in order to adjust the homogeneity of the B1 field in a beneficial manner. This ensues by an optimized current distribution to sub-systems of the transmission antenna of the Tx array. Electrical fields that can elevate the temperature of the tissue of the subject are also produced in the measurement subject (for example in the human body) by different current distributions among the transmission antennas. For example, an antenna of the type known as a birdcage antenna, that is formed by a number of coil elements, is known for use as a transmission antenna arrangement (an RF coil).
A measure of the heating due to RF absorption is the Specific Absorption Rate, (SAR). Predetermined SAR limits (that are predetermined, for example, in IEC standards) must be complied with.
In magnetic resonance apparatuses with magnetic field strengths of greater than or equal to three Tesla, a differentiation exists between a local SAR value (SARlocal) that is limited to a limited volume, and a global SAR value (SARglobal) averaged over an entire volume. Ideally the two SAR values SARlocal and SARglobal should approximately correspond, but in reality this is not the case at the aforementioned higher magnetic field strengths.
For transmission pulses that are used to examine the patient, the power is measured, evaluated with the aid of software, and used to estimate the global SAR value and the local SAR value. The SAR values (and thus also their estimates) are dependent on the employed transmission antenna or transmission antenna arrangement.
A direct determination of the electrical field is not possible, but it can be analyzed with the use of complicated simulation programs and simulation calculations.
Software programs (for example from the company “Remcon” (www.remcon.com)) can be used for simulation. This company in particular provides software for electromagnet analysis, for microwave simulations, for antenna simulations and for determination of radio wave propagation for biomedical applications.
Such programs employ elaborately generated human models, for example the model of a man designated as “HUGO” or the model of a woman designated as “IRENE”. Both models provide their respective tissue types in a three-dimensional data set.
Using such models, it is possible to determine the influence of a specific person or his or her tissue on an employed transmission antenna and to determine tissue-dependent variations of the B1 field and, from those, variations of the electrical field. From the modeled current distribution to the transmission antenna elements, it is possible to determine and to compensate an assumed (calculated variation of the B1 magnetic field and of the electrical field.
Since “HUGO” and “IRENE” describe only a specific person as a model, no better than a relatively imprecise compensation is achievable.